servo: Merge #7487 - Have PaintLayers own StackingContexts instead of the opposite (from mrobinson:paint-layer-upgrade); r=pcwalton

Previously, StackingContexts might have a PaintLayer. We switch the
ownership, for several reasons:

   * We want PaintLayers to potentially contain something other
     than a StackingContext soon.
   * We want to delay the creation of PaintLayers until the last
     minute, so that we can synthesize new layers for sandwiched
     content.

This commit also implements the second goal. Instead of creating
PaintLayers during layout itself, wait until we are sorting and
layerizing a completed DisplayList.

Source-Repo: https://github.com/servo/servo
Source-Revision: 8e0b010117889550effa3ad793e7ee8f9b692b40

servo/components/msg/compositor_msg.rs

@@ -35,7 +35,7 @@ impl FrameTreeId {
     }
 }
 
-#[derive(Clone, PartialEq, Eq, Copy, Hash, Deserialize, Serialize)]
+#[derive(Clone, PartialEq, Eq, Copy, Hash, Deserialize, Serialize, HeapSizeOf)]
 pub struct LayerId(pub usize, pub u32);
 
 impl Debug for LayerId {
@@ -59,7 +59,7 @@ pub enum LayerKind {
 }
 
 /// The scrolling policy of a layer.
-#[derive(Clone, PartialEq, Eq, Copy, Deserialize, Serialize, Debug)]
+#[derive(Clone, PartialEq, Eq, Copy, Deserialize, Serialize, Debug, HeapSizeOf)]
 pub enum ScrollPolicy {
     /// These layers scroll when the parent receives a scrolling message.
     Scrollable,